This project is focused on the elucidation of signaling paths stimulated by the MAPK Kinase Kinase MEKK3 and on MEKK3?s physiologic functions in cells. MAPK pathways have been shown to be essential to cell growth, differentiation and apoptosis. It is therefore critical to gain a better molecular understanding of component parts and of the targets of the individual MAPK pathways in normal and diseased states. We originally cloned MEKK3 and determined that it can activate all three major MAPKs, namely ERK, JNK and p38, albeit to various degrees. We discovered that MEKK3 activation arrests cell cycle progression. We generated a conditionally-activatable form of this kinase that responds to estrogen. When cell lines permanently transfected with this form of MEKK3 were induced with estrogen, the cells were blocked in transition through the G1 phase of the cell cycle. We demonstrated that this arrest is mediated via negative regulation of a necessary component of progression, namely cyclin D1. In addition we demonstrated that the arrest is mediated in a p38-dependent manner. These observations represent the first reports on functions of MEKK3. MEKK3 also reverses Ras-induced cell transformation and may serve to play an important role to arrest cell cycle progression during cellular stress or damage. - Signal transduction; cell cycle arrest; growth control; estrogen- inducible kinase; mitogen-activated protein kinases; MEKK3; NIH3T3.